Engine turbochargers normally operate at extremely high speeds of the order of 120,000 revolutions per minute. To obtain maximum power from the engine, it is essential that the speed of the turbocharger unit be as high as possible; the higher the speed, the greater the quantity of air supplied to the engine. However, the speed is subject to a highly critical threshold which must not be exceeded. If this occurs, bearing life will be drastically reduced by breakup of the lubricating oil film.
In predelivery testing of engines, it is desirable to measure the speed of the turbo at full load and to adjust the value by means of a bypass valve to ensure that the threshold is not exceeded. Since the turbocharger is an integral part of both the engine exhaust and intake systems, access for measurement by conventional methods is difficult.
In the method used until now, a magnetised nut is mounted on the turbo shaft and the speed of the magnetic field generated as the nut rotates is measured. This method is both time-consuming and unreliable since it is difficult to measure the speed of the field with the desired accuracy; the measuring device or probe, which consists essentially of a pickup, cannot be located sufficiently close to the shaft due to the size of the turbo.